PROJECT SUMMARY . Mitochondria play a pivotal role in regulating cardiac function in health and disease by regulating energy balance, biosynthetic processes, and cellular survival. Mitochondrial quality control and homeostasis are maintained in part through fusion and fission of mitochondria as well as through autophagic clearance or mitophagy of damaged mitochondria. Disruption of mitochondrial homeostasis has been closely linked to a host of acquired and genetic disease states, characterized by cardiomyocyte death at the cell level and overt cardiac dysfunction at a systemic level. Thus, clearly a thorough understanding of the molecular regulators of mitochondrial homeostasis in the heart is critical for reducing cardiac dysfunction related mortality. Our preliminary data demonstrate that the mitochondrial E3 ubiquitin ligase, Mulan, is a novel regulator of mitochondrial dynamics and mitophagy in mammalian hearts. Mulan participates in the maintenance of mitochondrial integrity and function. While Parkin, a cytoplasmic E3 ligase, has been extensively studied in mitophagy, Mulan's proposed role as a mitochondrial injury sensor, mediator of mitophagy, and inducer of cell death is both novel and a significant departure from the current state of understanding in the field. Given that Mulan is upregulated in Parkin null mouse hearts, we postulate that Mulan's upregulation may serve to compensate for the loss of Parkin, thus maintaining the basal cardiac function of Parkin null mice. Employing state-of-the-art techniques, we will investigate the intricacies of protein-protein and protein-substrate interactions by Mulan as well as how modulation of Mulan's expression fundamentally alters mitochondrial dynamics. Ultimately, our data may provide the first glimpse of an unrecognized, yet significant role of Mulan as a ?resident mitochondrial injury-sensor? in the heart. Furthermore, we intend to define the distinct contribution of Mulan in mitophagy, that is independent from Parkin, as well as Mulan's role in mitochondrial function, dynamics, and cell death in the heart. Given the emerging importance of mitochondrial dysfunction in cardiac pathology, the findings from our proposal will help define the role of Mulan as a key mitochondrial injury-sensor. In addition, the proposed studies will have broad implications for the understanding and treatment of heart disease.